Identification of Groundwater Nitrate Contamination from Explosives Used in Road Construction: Isotopic, Chemical, and Hydrologic Evidence

Explosives used in construction have been implicated as sources of NO3- contamination in groundwater, but direct forensic evidence is limited. Identification of blasting-related NO3- can be complicated by other NO3- sources, including agriculture and wastewater disposal, and by hydrogeologic factors affecting NO3- transport and stability. Here we describe a study that used hydrogeology, chemistry, stable isotopes, and mass balance calculations to evaluate groundwater NO3- sources and transport in areas surrounding a highway construction site with documented blasting in New Hampshire. Results indicate various groundwater responses to contamination: (1) rapid breakthrough and flushing of synthetic NO3- (low delta N-15, high delta O-18) from dissolution of unexploded NH4NO3 blasting agents in oxic groundwater; (2) delayed and reduced breakthrough of synthetic NO3- subjected to partial denitrification (high delta N-15, high delta O-18); (3) relatively persistent concentrations of blasting-related biogenic NO3 derived from nitrification of NH4+ (low delta 15N, low delta 18O); and (4) stable but spatially variable biogenic NO3 concentrations, consistent with recharge from septic systems (high delta N-15, low delta O-18), variably affected by denitrification. Source characteristics of denitrified samples were reconstructed from dissolved-gas data (Ar, N-2) and isotopic fractionation trends associated with denitrification (Delta delta N-15/Delta delta O-18 similar or equal to 1.31). Methods and data from this study are expected to be applicable in studies of other aquifers affected by explosives used in construction.